1. Field of the Invention
The present invention relates to an information transmission method and the system. More particularly, the present invention relates to an information transmission method and the system tolerant of both of rain and fog. In addition, the present invention relates to an optical space transmission system and the method in which degradation of transmission quality and circuit disconnection are decreased.
2. Description of the Related Art
First, conventional technologies of a first aspect will be described.
FIGS. 1A and 1B show block diagrams showing conventional optical space transmission apparatuses.
As for FIG. 1A, in a sending station 1, a signal to be transmitted is input from an input terminal 3. Then, the signal is converted to a signal format for optical transmission by a signal converter 5. The optical transmitter 6 sends the optical signal to the air. The optical signal is received by an optical receiver 7 in a receiving station 2 and converted to an electrical signal. The electrical signal is converted into an information signal by a signal converter 8 and output from an output terminal 4.
As for an example shown in FIG. 1B, in the sending station 1, a signal to be transmitted is input from an input terminal 3. Then, the signal is converted to a signal format for optical transmission by a signal converter 5. The optical transmitter 6 sends the optical signal to the air. The optical signal is received by an aperture enlarged by a receive aperture enlarger 9 in the receiving side 2. Then, the optical signal forms an optical image on a receive aperture surface of an optical receiver so that the optical signal is received by the optical receiver 7. The optical signal received by the optical receiver 7 is converted to an electrical signal. The electrical signal is converted into an information signal by a signal converter 8 and output from an output terminal 4.
According to the above-mentioned configuration, when the optical signal transmitted from the optical transmitter is sent to the receiving station via the air, intensity of receive light may fluctuates (spot dancing, beam bending, scintillation) in the receiving station since the refractive index of the air changes due to fluctuations of air temperature and humidity so that propagation path of the light beam fluctuates. Thus, there is a problem in that circuit quality degradation or circuit disconnection occurs due to the above-mentioned fluctuations even in good weather.
It is possible that the effect of the fluctuations can be decreased by enlarging the receive aperture as shown in FIG. 1B. However, it is difficult to prepare a large aperture due to lack of image formation technique and limitation of the size of the apparatus. In the configurations shown in FIGS. 1A and 1B, although electrical/optical (E/O) converters and optical/electrical (O/E) converters are provided, they are not shown in the figures.
Next, conventional technologies of a second aspect of the present invention will be described.
Conventionally, a radio transmission and an optical space transmission are performed separately. The conventional information transmission method will be described with reference to FIGS. 2A and 2B. FIG. 2A is a block diagram of a conventional radio transmission system, and FIG. 2B is a block diagram of a conventional optical space transmission system.
In a conventional radio transmission system, data (information signal) is input from an input terminal in a sending station 1001. The signal is converted into a radio signal of radio frequency band by a radio modulator 1002, and sent to the air via a radio transmitter 1003 and an antenna 1004.
In a receiving station 1005, the sent radio signal is received by a radio receiver 1007 via an antenna 1006, and the received signal is demodulated into original data (information signal) by a radio signal demodulator 1008.
On the other hand, according to a conventional optical space transmission system shown in FIG. 2B, data (information signal) is input from an input terminal in a sending station 1009. The signal is converted into a signal format for optical communication by a signal converter 1010. After that, the signal is converted into an optical signal by an electric/optical (E/O) converter 1011 and the optical signal is sent to the air by an optical transmitter 1012.
In a receiving station 1013, the sent optical signal is received by an optical receiver 1014. Then, the signal is converted into an electrical signal by an optical/electrical (O/E) converter 1015. After that, the signal (information signal) is converted into original signal by a signal converter 1016.
Accordingly, since either radio transmission or optical transmission is used for conventional information transmission, the sending station and the receiving station in a radio transmission system only have functions of sending and receiving a radio signal respectively, and the sending station and the receiving station in a optical space transmission system only have functions of sending and receiving an optical signal respectively.
However, for realizing high capacity information transmission by the conventional information transmission system, there is a problem in that keeping reliable availability and long distance transmission are difficult.
That is, for realizing high capacity information transmission in radio transmission, since wide bandwidth becomes necessary, the radio frequency band becomes usually high frequency band (submillimeter wave, millimeter wave). This frequency band is resistant to fog, however, very large propagation loss occurs due to rain in this frequency band. Thus, it is necessary to provide a large margin considering attenuation due to rain.
In addition, as for optical space transmission, it is resistant to rain, however, remarkably large propagation loss occurs due to degradation of visibility by fog. Therefore, it is necessary to provide a large margin considering attenuation by fog.
As mentioned above, the conventional information system in which only one of radio transmission and optical space transmission is performed is not resistant to rain or fog. Thus, it is necessary to provide a margin considering rain or fog so that keeping reliable availability or long distance transmission are difficult.